The present invention relates, in general, to a system and method for inductively converting variable-speed mechanical power into constant-frequency electrical power and, more specifically, to a system and method for controlling the load bus voltage of a variable-speed, wound-rotor induction machine to provide electrical power at a constant-frequency.
There has been an increasing interest in non-fossil fuel energy sources as a consequence of the escalating cost and political ramifications associated with a reliance on foreign oil. Wind turbine generators (“WTGs”) are one of many current sources that provide an environmentally-friendly, non-polluting energy source. WTGs are variable-speed current sources that convert mechanical energy into electrical power. More particularly, in WTGS, mechanical power, resulting from the wind-driven rotation of generator turbine blades, is converted into current. Conventionally, the turbine blades are mechanically-coupled to a rotor assembly of an induction-type motor generator. As the blades rotate, so does the rotor assembly.
Corresponding electrical windings are provided on the shaft of the rotor assembly and on a stator assembly. Rotation of the rotor shaft within the rotor assembly causes rotation of the rotor assembly windings and induces current flow in the stator assembly windings. The induced current can be converted and phased as necessary. The voltage also can be stepped-up (or stepped-down), e.g., using a transformer, to produce electrical power at a desired voltage.
Because the rate of rotation of the WTGs depends on the force of the wind, the blades and rotor shaft rotate at a variable speed due to the changing wind force and wind velocity. Variable rotational speeds can be problematic, however, because, when delivering electrical power to a load, especially when delivering power to an alternating current (“AC”) utility grid, current at a constant-frequency, such as 50 or 60 Hertz (Hz), is desired.
Conventionally, some power generators or power generating systems deliver electrical power to a load from the stator of the wound-rotor induction machine generator (WRIMG), using an electrical “cascade” arrangement to provide an excitation current to the windings of the rotor of the WRIMG through slip-rings. Historically, a “cascade” arrangement refers to a rotating drive arrangement in which the rotor assembly of a WRIMG is fed from the stator assembly of an auxiliary induction machine that is mounted on the same rotor shaft. More particularly, the rotating shaft of a wind turbine is mechanically coupled to the rotor assembly of a WRIMG thereby providing mechanical input power to the WRIMG. The electrical cascade that supplies the rotor windings is essentially a controlled variable-frequency power source.
Referring to FIG. 1, mechanical power from the wind turbine shaft (not shown) is transferred to the rotor assembly 16 of the WRIMG 15. The transferred mechanical power drives the rotor assembly 16, causing it to rotate. The rotor windings 13 disposed on the rotor assembly 16 induce current in the corresponding stator windings 19, which are disposed on the stator assembly 17. The induced current in the stator windings 19 is then delivered by a stator bus 21 to a load such as an AC utility power grid 18.
For the conventional system 10 shown in FIG. 1, the AC utility power grid 18 is directly coupled to the stator windings 19 of the WRIMG 15. Therefore, AC grid voltage of the AC utility power grid 18 determines (by virtue of its magnitude and frequency) the actual level of excitation of the machine 10 for each phase and the synchronous speed of the machine 10.
Such machines 10 are controlled, generally, by controlling the rotor current in a continuously rotating reference frame that is determined by the instantaneous stator voltages. A controlled current is delivered to the rotor windings 13 from the electrical cascade at slip frequency, i.e. the difference between the electrical frequency ω of the stator assembly 14 and the (electrical) rotation frequency ωr, or speed, of the rotor assembly 16. Electrical rotation frequency is the rotational speed of the rotor assembly 16 (radians/second) multiplied by the number of pole-pairs in the machine 10.
The rotor current, in turn, largely determines the stator current and, hence, the power delivered from the stator assembly 17 to the AC utility power grid 18. Therefore both the instantaneous phase of the rotor current in the rotating reference frame, as well as its magnitude, must be regulated in order to deliver a desired power to the AC utility power grid 18 with a desired power factor.
The rotor terminal power, or “slip power”, is approximately proportional to the per-unit slip frequency, i.e., ((ω−ωr)/ω), multiplied by the stator terminal power. Therefore the power rating of inverters 12 and 14 is generally a fraction of the total power delivered to the AC utility power grid 18.
In the conventional system 10 shown in FIG. 1, the rotor assembly 16 is electrically-coupled to a cascaded rotor inverter 12 and grid inverter 14 via a rotor bus 22. Electrical power is transferred between the rotor assembly 16 and the AC utility power grid 18 via the cascaded inverters 12 and 14. Conventionally, the rotor inverter 12 is capable of delivering current to the rotor assembly 16 at the maximum rated current over a range of slip frequencies, including zero frequency.
The slip frequency, which is also the electrical frequency appearing at the rotor terminals or “slip rings”, will assume values within a limited range depending on the rotor speed range and the associated stator frequencies imposed by a controller.
At any point in time, the variable speed of the rotor assembly 16 can be at, above or below synchronous speed. During super-synchronous operation, corresponding to negative slip conditions, stator power and rotor power flow out of the machine 10 to the load via the stator bus 21 and the rotor bus 22, respectively. In contrast, during sub-synchronous operation, corresponding to positive slip conditions, stator power flows out of the machine 10 via the stator bus 21. However, instead of rotor power flowing out of the machine 10, power from the AC utility power grid 18 flows into the rotor assembly 16 via the rotor bus 22.
As a result, some portion of the stator power is returned to the machine 10 through the rotor assembly 16. Disadvantageously, some prior art systems restrict machine 10 operation either to a super-synchronous mode of operation or to a sub-synchronous mode of operation. Thus, in these instances, the topologies for the grid inverters 14 and rotor inverters 12 are structured and arranged to provide uni-directional power flow only.
So, for example, if the machine 10 topology were structured for sub-synchronous operation, when operating speed of the machine 10 is super-synchronous, rotor assembly 16 power is prevented from flowing out of the machine 10. If, on the other hand, the machine 10 topology were structured for super-synchronous operation, when operating speed of the machine 10 is sub-synchronous, there would be no way for power from the AC utility power grid 18 to flow into the rotor assembly 16.
FIG. 2 provides an alternative conventional topology that does not require use of an electrical cascade for rotor excitation. This full power, double-conversion system 20, typically, does not include external connections to the rotor assembly 16, which may have internally short-circuited windings or be of the “cage”-type of construction. Hence, all of the electrical power output comes from the stator assembly 17.
As shown in FIG. 2, the stator windings 19 disposed on the stator assembly 17 are electrically-coupled to the AC utility power grid 18 through a full-power-rated, bi-directional power converter 25. The power converter 25 includes an AC-to-DC stator inverter 11 and a DC-to-AC grid inverter 14, and is capable of transferring power between the stator windings 19 and the AC utility power grid 18 even if the frequency of the stator terminal voltages differs from the frequency of the voltage on the AC utility power grid 18.
Such machines 20, generally, are controlled using “field-oriented control” techniques. Field-oriented control, or “vector power control”, controls torque and current by interacting, i.e., crossing, an impressed stator current vector with a rotor flux vector that is generated by the inductance of the rotor assembly 16 and the stator assembly 17. As a result, field-oriented control applies a variable, rotating frame of reference, i.e., the rotor flux vector, to decouple the flux-producing stator current from the torque-producing stator current.
Thus, vector power control provides the proper and most efficient alignment or angular relationship, i.e., orthogonal or a 90-degree orientation, of a desired, flux-producing portion of stator current that is in a perpetually rotating flux field and a desired, torque-producing portion of stator current. More specifically, a desired rotor flux angle, which is defined by the angle of the flux field coordinate system with respect to a stationary frame of reference, is periodically determined and used to adjust the stator current.
The full-power, double-conversion system 25 provides an interface between the AC utility power grid 18 and the WRIMG 15. Consequently, the excitation of the WRIMG 15 is not directly influenced by the voltage of the utility power grid 18. However, a major disadvantage of full-power, double-conversion schemes and field-oriented control is the relatively high cost of full-power-rated power converters. Accordingly, it would be desirable for an alternative system that provides an interface between the AC utility power grid 18 and the WRIMG 15; that does not use field-oriented control or full-power-rated power converters; that requires lower total inverter power; and that regulates slip to maximize output power capability over a selected frequency range.